1. Field of the Invention
The invention relates to a method of forming X-ray images from at least two series of projection data sets successively acquired along different trajectories, a respective 3D data set being formed from each series of projection data sets. The invention also relates to an X-ray device which is particularly suitable for carrying out this method.
2. Description of the Related Art
A method and a device of this kind are known from EP 860 696 A2. Therein, two series of projection data sets are acquired along two semi-circular trajectories by means of a C-arm X-ray device, said trajectories extending at an angle of 60xc2x0 relative to one another. Each series of projection data sets forms a respective 3D data set wherefrom a respective reconstruction image can be formed. Because a single 3D data set does not contain adequate data for a complete and correct reconstruction and artefacts occur during the reconstruction, the two (or more) 3D data sets are combined by weighted addition. The desired images are formed from the resultant data set by reconstruction; artefacts occur to a lesser extent in said images.
The acquisition of the series of projection data sets along the different trajectories normally takes place successively in time. For optimum compatibility of the projection data sets, or the 3D data sets to be formed therefrom, during the subsequent combination and reconstruction, it would be necessary for the object to be examined, for example a patient, to remain motionless during the data acquisition. In particular the position of the object to be examined should always be identical during the acquisition of the individual series of projection data sets and any translatory or rotary motions of the object to be examined should be as small as possible. However, because this can hardly be completely achieved during a practical examination of a patient, it is also known to reproduce, for example a phantom member in the X-ray images during the acquisition of the projection data sets; such a phantom can subsequently be used for fine adjustment so as to achieve matching 3D data sets. This operation is performed by a user.
Therefore, it is an object of the invention to provide a method which enables combination of 3D data sets without it being necessary for a user to perform a fine adjustment operation. It is also an object to provide an X-ray device which is suitably constructed for this purpose.
These objects are achieved by means of a method as disclosed in claim 1 and by means of an X-ray device as disclosed in claim 6.
The invention is based on the recognition of the fact that the same object to be examined is reproduced in all 3D data sets and that, therefore, individual structures can be traced in all 3D data sets. According to the invention this fact is used so as to select the voxel image values of at least one sub-volume in a first 3D data set and to search for these values in the other 3D data sets in order to derive therefrom a transformation rule describing a translatory or rotary motion, if any, occurring between the formation of individual 3D data sets. Generally speaking, the sub-volume V2 is then selected automatically. The search in the other 3D data sets for voxels selected in a first 3D data set utilizes a suitable similarity measure for iteratively finding the corresponding voxel in the other 3D data sets.
Depending on the desired accuracy, this method can be performed with the appropriate number of voxels which should be distributed as well as possible throughout the entire volume represented by the 3D data set. The transformation rule or transformation rules found are then used to correct for motions of the object to be examined, to achieve quasi matching of the 3D data sets, to combine them so as to form a complete data set and to form the desired images therefrom. According to the method of the invention the foregoing can be realized without utilizing a phantom object or other markers reproduced in the X-ray images; the method can be performed automatically, that is, without interventions by a user.
In order to determine the transformation rule, several voxels located in respective sub-volumes of a 3D data set and/or individual voxels containing significant image information are advantageously selected in conformity with the claims 2 and 3.
Preferably, the functions indicated in claim 4 are used as a similarity measure. However, other possibilities are also feasible.
The method according to the invention is used primarily for a C-arm X-ray device, but can also be used in a computed tomography device; the invention can also be used notably in an X-ray device or a computed tomography device involving a conical X-ray beam.
Claim 6 discloses an X-ray device according to the invention which includes an X-ray source, an X-ray detector, a reconstruction unit and an arithmetic unit.